1. Fields of the Invention
This invention relates to a water based ink capable of decolorizing or discoloring written or printed letters or images through an organic solvent or heating, which is used for a writing instrument such as roller ball, marker, etc., printing ink, ink jet.
2. Description of the Prior Art
Lately, as an ink capable of decolorizing or discoloring, there have been developed and have practically been used an ink of aqueous dye type utilizing an oxidation and reduction reaction (JP-B-54-22344) and an ink of oily dye type using leuco dyes (JP-B-2-29102). Since the above described ink is an ink of dye type, however, written lines thereof are inferior in water proof property and light proof property and when writing on a paper, there occur blurring or bleeding through it. Furthermore, in the case of an ink of the type utilizing an oxidation and reduction reaction, such a problem arises that writing cannot again be carried out because a decolorizing solution containing a reducing agent is excessively coated during decolorizing and the reducing agent remains on a decolorized site. As a prior art technique, an ink capable of decolorizing by irradiation of infrared rays, has been known (JP-A-6-73319), but this ink meets with a problem that the ink or written lines tend to fade.
As a method for solving the above described problem, it can be considered to use a pigment for a colorant. In general, an ink using a pigment is excellent in water proof property as well as light proof property of written lines, so that the blurring or bleeding through is hard to occur, as well known in the art. As a method of pigmentizing using a leuco dye, there are proposals to make pigments by applying methods for preparing tonors to be used for printers or copiers (JP-A-10-88046, JP-A-10-287081). In order to decolorize written lines of the ink, based on JP-A-10-287081, the paper for writing is immersed in a special remover, followed by heating, and the decolorizing is thus rendered possible. This teaches that it is impossible to readily effect the decolorizing. The ink based on JP-A-10-88046 comprises a leuco dye, developer, decolorizer and resin, in which the leuco dye and developer are interacted and are in color-developed state. This decolorizer is a material having such a property as preferentially combining with the developer during melting the resin or dissolving the resin with a solvent, whereby the decolorizing is rendered possible with an organic solvent and/or by heating.
In addition, as an ink for a writing instrument to which the above described technique is applied, a water based ink composition for a roller ball has been proposed (Japanese Patent Application No. 249544/2000). The above described pigment has an advantage that repeated writing can be carried out on a decolorized site, since a leuco dye and decolorizer are coated with a resin and are not brought into direct contact with each other.
However, the above described colorant is generally prepared by mixing and kneading a color former, developer, decolorizer and resin to obtain a lump of product and then pulverizing in gaseous phase by means of a jet mill to give the colorant. The colorant prepared by the above described method has the following disadvantages that:
1) it is difficult to obtain a colorant with a particle or grain diameter of at most 3 μm in view of the structure of a pulverizer, during which an ink prepared therefrom meets with problems on the precipitation and adhesiveness property of the colorant,
2) the grain diameter is not uniform, so writing feel is bad,
3) the surface of the colorant is not smooth, so diffused reflection of light takes place to cause white turbidity seen as pale color,
4) since there is a color-developing component even in the inside of the colorant, not related with the color tone of the ink, the color-developing component is present in an amount than required,
5) because of the above described items 3) and 4), the ratio of the color-developing component is increased, resulting in increase of the cost and
6) since the decolorizer is also contained in the colorant, the resultant water based ink encounters a problem of fading of the ink color during passage of time.
The prior art ink of a dye type, capable of decolorizing or discoloring, is inferior in water proof property and light proof property and when writing on a paper, there occurs blurring or bleeding of the ink through the paper. Moreover, such a problem arises that writing cannot again be carried out where the decolorizing has been carried out. In the case of an ink of a pigment type, there is also a problem that fine granulation is difficult, writing feel is bad and color tone is too pale.